Instigate GUI Toolkit is an Abstraction for multiple GUI Toolkits allowing to write single GUI code which can run on multiple platforms and with multiple GUI back-ends. Now it supports QT and gtkmm back-ends on Linux. Work on GTK+ and EWL is ongoing.
ACDK - Artefaktur Component Development Kit - is a platform independent C++-framework similar to Java or C#/.NET for generating distributed and scriptable components and applications.